The dynamic range of analog information signal-processing circuits is inherently constrained by circuit noise, which obscures the smallest information signals, and power supply limitations, which limit the largest information signals that can be processed accurately. Many applications, especially portable, battery-powered audio products, require wide dynamic range and simultaneously low-power operation from low-voltage power supplies.
A particular class of applications involves conditioning the output voltage from a transducer or sensor. It is often advantageous to amplify the output voltage from such a source prior to further signal processing. It is also often advantageous to terminate such a source with a specific load impedance to maximize its signal-to-noise ratio and/or tailor its frequency response. These functions are typically accomplished by a preamplifier. In systems operating from low power-supply voltages, it is often desirable for the circuit to perform further signal processing on currents representative of the preamplifier output, rather than voltages. As is well known in the art, many current-mode, signal-processing circuits have been developed that minimize signal-related voltage excursions in the circuit. Such circuits, if designed to operate in class AB, also consume little power supply current under quiescent conditions. This makes them particularly desirable for battery-powered applications.
When such a signal-processing system is implemented in integrated circuit form, all sections of the circuitry typically operate from a single pair of power supply terminals. In this case, the power supply voltage of the entire IC must be set to accommodate the maximum preamplifier output voltage necessary to achieve the desired dynamic range. In systems with wide dynamic range requirements, this results in other parts of the circuitry, such as current-mode signal processing circuits, operating at a higher-than-necessary power-supply voltage.